Treating it like the monorail in DzC - i.e. comes in on one of the perpendicular edges (low orbit) of the table and moves 12" (12" on a 6x4 table, dropping to 8" on a 4' by 4' table - so it will transit the whole table in 6 turns) every turn as it moves across the table. It counts as a critical location, and a small cluster for scoring, on turns 3 and 5, it will be alongside small space stations, where, at each station, player with troops on the stations can move a max of 2 infantry (only) tokens on to it (max of 4 infantry tokens per player on the liner).Additional clusters for that mission (at Skirmish level), could be 2 medium sectors of 1 commercial, 1 military, 1 industrial, placed like the mediums in Grid Control. At Clash level+, add in 2 more medium stations.

VIP: Any ship of Tonnage M or higher within 6" of the cruise liner has the option of shielding the liner at the cost of a minor spike (think of it like deliberately trying to drown out the liner's signature with an energy spike). In practice - player A has a ship with 6" of the liner, and player B announces which weapon systems will attack the liner - player A can re-direct 1 weapon system targeting the liner per minor spike they are willing to take.

Boarding is that you could board the liner on turns 3 and 5, only at the stations it stops at.

I like this idea, and in general, I think this game could use some more diverse scenarios.

The VIP rule might need to be tweaked a little. If you can shield a civilian vessel, then why wouldn't you be able to do the same in a normal battle? A redirect could be stronger than the special defense frigates in the right situation. In fact, some factions are specifically better because of their frigates (which is probably why you invented the rule... to even the playing field). You can obviously invent some fluff to make it work.

The boarding rule seem very limiting. I realize that it's based on the monorail scenario, and that' a key component. But if a strike carrier/troopship can board a hostile space station, you'd imagine that it could hostile board a civilian ship. (again you could fluff it away).

Given the current rules, you've laid out, you might need to up the VP value of the cruise liner, or specify that the other clusters/stations are worth less. If there are any other critical locations/scoring sectors, then it seems too easy to ignore the cruise liner and blast your opponent if they try to cap it.